Tachogenerator for speed control of electric motors

ABSTRACT

A motor has been proposed in German Offenlegungsschrift DE-OS No. 2 730 142. The windings of the tacho winding are disposed here next to the stator winding in the flat air gap of the main magnet, and the current changes in the statorwinding induce therefor in the tachowinding voltages interfering with a precise working control. On the other hand would a disposition of the tacho generator at a different position of the motor increase its axial length considerably, especially when a magnetic shielding between the motor and the tachogenerator is provided. Therefor, it is a purpose of the present invention to provide a motor of the kind initially cited, which with compact construction and especially short length in direction of its rotation axis provides a low interference tacho signal remaining the same or improving with increasing age of the motor, for example in order to improve in any case, but at least to keep the same the ganging flutter of a speed controlled direct drive motor for a record player.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of another applicationfiled July 9, 1980, and bearing Ser. No. 166,806 now abandoned. Theentire disclosure of this latter application, including the drawingsthereof, is hereby incorporated in this application as if fully setforth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tachogenerator for speed control ofelectric motors and in particular for brushless d.c. motors.

2. Summary of the Invention

The present invention provides a tachogenerator for speed control ofelectric motors having an axial bearing for compensating the axialforces acting on the motor. A flat multipole permanent magnet is solidlyconnected to the rotor of the motor and has a number p of pairs ofmagnetic poles on its surface uniformly arranged on a circle of diameterd for the pole centers. A circular wave winding is solidly connected tothe stator and separated from the permanent magnet by a flat air gap ofa width g and has a number p of angular substantially repeat units u ofthe wave winding. The number of pole pairs p follows the formula

    p=(0.5 to 1.5)·(d/g)

wherein d is the diameter of the centers of the poles, g is the width ofthe air gap and d and g are measured in the same units.

Preferably the number p of pole pairs is equal to from about 0.8 to 1times the ratio of the diameter d divided by the width g, with d and gagain being measured in the same units. Preferably the diameter d is atleast about 10 mm and at most about 100 mm and more preferred is a rangeof from about 30 mm to 50 mm. Preferably the width g of the air gap isat least about 0.2 mm and at most about 2 mm and more preferred is arange for g of from about 0.5 to 1 mm.

Preferably the number p of pole pairs of the multipole permanent magnetis equal to from about 1 to 2 times the number p of pole pairscorresponding to the maximum induced voltage at the frequency desired.

It can be advantageous for the tachogenerator to comprise in addition tothe wave winding a corresponding return winding running in oppositedirection to the wave winding for compensating the effect of the wavewinding on a magnetic field component in axial direction running throughthe circle of the wave winding. The wave winding can be a meanderwinding. The meander winding can be provided by a conductor disposed ona plastic disc by photolithographic techniques.

Also, a soft magnetic material preferably in magnetic shield form can bedisposed on the side of the wave winding opposite to the side facing themultipole permanent magnet.

The wave winding is preferably attached to a support tube of the rotor.The multipole permanent magnet is preferably attached to a soft magneticbacking solidly connected to the rotor. The multipole permanent magnetcan be a so called rubber magnet and this rubber magnet can beadhesively attached to a soft magnetic material. A preferred electricmotor is a two phase brushless d.c. motor which can comprisesuccessively a north pole magnet zone of 120° electrical, a south polemagnet zone of 120° electrical and a remaining zone of 120° electricalhaving a magnetization which generates in a magnetically active sectionof the stator winding at a relative motion a substantially zero voltage.

In a preferred embodiment of the invention a brushless d.c. motor with aflat air gap is provided wth a tachogenerator for speed control. Atleast one stator coil is disposed for generating an electromagnetictorque and a rotor magnet arrangement is disposed at a certain distancefrom the stator coil around a rotation axis. The rotor magnetarrangement is preferably an axially polarized magnet ring. An axialbearing is provided for compensating the axial forces exerted on therotor, especially as disclosed in German patent application No.P2,730,142.4-32 or, respectively, U.S. Pat. No. 4,211,963. In the spacebetween the rotation axis and the rotor magnet arrangement there isdisposed a flat multipole permanent magnet and connected to the rotor ofthe motor in a solid angular relation.

Opposite to the rotor and separated by an air gap which is at leastnearly parallel to the air gap of the motor is disposed a wave ormeander winding at the stator. A voltage is induced in the wave ormeander winding upon operation of the motor by the flat multipolepermanent magnet and the frequency of the voltage is proportional to thespeed of the motor.

The air gap of the motor on the side opposite to the side facing therotor magnet can be surrounded by soft ferromagnetic materials forincreasing magnetic field strength and these soft ferromagneticmaterials and the stator are provided with a feedthrough for theconnectors of the wave or meander winding.

Preferably the number p of pole pairs of the multipole flat permanentmagnet and the number of the magnetically active sections of the wave ormeander winding are selected such that in a diagram, showing the numberof pole pairs on the abscissa and the voltage u induced in the wave ormeander winding at a constant speed of rotation with a constantfrequency n, the number p of pole pairs is in the region of the voltagemaximum and preferably above said maximum.

The air gap of the motor can be bordered on the side away from the rotormagnet by stationary soft magnetic materials and these soft magneticmaterials can be formed as plates, they can be pierced in their middleby a bearing tube and they can be together with the bearing tubesurrounded by plastic.

The plastic can also support the stator winding and at the same timeprovide insulation.

The size of the maximum of the voltage u depends on the frequency, butdoes not shift with frequency. Preferred materials for the multipolemagnet are hard magnetic materials with high coercitive force such asplastic barium ferrite, sintered barium ferrite or samarium cobalt. Whenthe magnetic pole material demagnetizes, then a lower pole pair numberis preferred. By the arrangement of the tachogenerator in the spacebetween the rotation axis and the rotor magnet there results a compact,relatively small arrangement wherein stray fields from the statorwinding or from the rotor are less effective in this region as in theair gap of the motor and if desired the stray fields can also bescreened by iron sheets or the like in the usual fashion.

Since by experience the axial bearing of the rotor sinks somewhat inwith increasing age and therefor the air gap of the tachogeneratordecreases, the output voltage increases with increasing age of the motorresulting in an improvement of the uniform running properties withincreasing age.

Further details and advantageous embodiments of the invention followfrom the below described and in the drawing shown embodiment, which inno way is to be considered as a limitation of the invention, as well asfrom the subclaims.

It is shown in

FIG. 1 a section through a preferred embodiment of a motor according tothe invention, as seen along section line I--I of FIG. 2,

FIG. 2 a plan view onto the stator winding of the motor according toFIG. 1, as seen about along the section line II--II of FIG. 1, whereindetails of the shaft and its bearing are left out,

FIG. 3 a representation on a larger scale when compared to FIG. 1 and 2and as seen about along the line III--III of FIG. 1, wherein again theshaft and the bearing arrangement of the motor are not shown, and

FIG. 4 a show picture for explaining a preferred embodiment of thetachogenerator.

FIG. 5 shows a diagram of the relative voltage plotted versus the polepair number p.

FIG. 6 shows a diagram of the relative voltage plotted versus the airgap in mm. Curve A is for a meander winding on a plastic disc and curveB is for a meander winding on a plastic foil supported by a softmagnetic iron sheet.

In principle, the motor 10 of FIG. 1 and 2 is of the same constructionas the motor according to FIG. 8 and 9 of German OffenlegungsschriftDT-OS No. 730,142 or respectively U.S. Pat. No. 4,211,963 wherein thecoil variant 141 according to said FIG. 9 is employed. Therefor, toavoid superfluous lengths expressly reference is made to the completedescription and drawing there.

The stator designated as 11 is provided with a basis plate 12 from amagnetically soft ferromagnetic material, typically iron, which servesas a magnetic short circuit and which carries at the same time the fourstator coils 13, 14, 15, 16, an axial bearing 17 and a bearing tube 18for the rotor shaft designated as 19. The bearing tube 18 is as is usualin slow running motors, formed as a friction bearing, but could ofcourse also be provided with antifriction bearings. The axial bearing 17is also a friction bearing and is provided with a plastic disc 19, whichrests against the trace spherical cap 20 of the shaft 9. Experience hasshown that in the course of time this trace spherical cap sinks somewhatinto the plastic disc 19, for example by 0.2 . . . 0.3 mm, and thiscircumstance results in an improvement of the quality of the motor inthe present invention as is shown in the following. For adjustment ofthe axial bearing 17 a set screw 23 is provided.

As shown, the basis plate 12 is surrounded with an injection moldedplastic piece 24, which

(a) attaches the bearing tube 18 safely by penetrating into its ringslots 25, 26, 27,

(b) catches the bolt 23 of the axial bearing 17 in its central bore,

(c) serves a guide for the hard to turn set screw 23 provided as a partsurrounded by injection molded material,

(d) supports the coils 13 to 16 through an upper collar 29,

(e) shows hollow spaces 32, 33 comimg from protrusions in the injectionmolding tool for the fixation of the stator coils,

(f) supports a conducting plate 34, which serves as a base for a Hallgenerator or another galvanomagnetic sensor element, by way of twoplastic rivets 36, 37, which after the mounting of this conducting plateare made by hot heading of protrusions of the molded piece 24.

In addition, the molded piece 24 and the basis plate 12 comprise afeedthrough 38 for passage of the connecting wires 39 of atachogenerator designated in total as 40. The wires 39 are conducted tothe conducting plate 34 and are there soldered.

The basis plate 12 and the molded plastic piece 24 including the fourcoils 13 to 16 form together a compact stator 11 including a magneticshort circuit for the rotor designated as 44 of the motor 10. Thisconstructive concept for the stator is of course not only for anarrangement according to the stem patent, but also for all motorprinciples with standing magnetic short circuit and planar air gapinventively valuable.

On its upper side the conducting plate 34 is provided with a printedcircuit and carries further electrical devices 45, for example the powertransistors for the control of the coils 13 to 16, resistors, etc. It isadapted in its form to the coils 13 to 16 and extends with a tongue 46far into the air gap 47 of the rotor 44 in order to provide a largesignal at the Hall-generator 35, compare FIG. 1. The conductor plate 34for example can receive the complete control circuitry of the motor inthe form of so called integrated circuits. Then only a few feed wiresare necessary to the conducting plate for the operational functioning.If the rivet stumps 36, 37 are furthermore supplemented by disengageablefasteners such as bolts, then such an embodiment of the conducting platerepresents a very advantageous arrangement for manufacturing andservice. When the disengageable fasteners are disposed from the outsideas separately disengageable (for example the bolts are radially outsideof the rotor 44, wherein the plate 23 or 73 radially correspondinglyprotrudes), then the motor can be separately produced and the completeelectronic circuitry can in the following be additionally mounted in asimple way, which is of equal importance for servicing. Such anembodiment of the conducting plate is equally inventively valuable forother than the motor constructions according to the stem patentapplication, that is in general for brushless d.c.-motors, especiallythose with a planar air gap.

The stator coils 13 to 16 are according to the teachings of the GermanOffenlegungsschrift DT-OS No. 52,730,142 or, respectively, U.S. Pat. No.4,211,963 wound about as a pentagon and they are disposed equidistantly.The rotor 44 is four pole, that is it is magnetized as shown in FIG. 8of the German Offenlegungsschrift DT-OS No. 2,730,142 or, respectivelyU.S. Pat. No. 4,211,963. It comprises a magnetic short circuit plate 50from iron, which is flanged at a bush 51, which bush in turn is pressedto the shaft 19 and it is thereby solidly connected to the same. Amagnetic ring 52 from an oxide magnatic material is adhesively attachedto the magnetic short circuit plate 50 and is axially magnetized asexplained.

The magnetic ring 52 forms in its interior a hollow space 53 fordisposing the tachogenerator 40 in a space saving way. Thetachogenerator comprises a magnetic plate 54, which advantageously canbe a so called rubber magnet, that is a mixture of magnetically hardferrites and of an elastomer. This magnet plate 54 is directly attachedto the magnetic short circuit plate 50 and rotates therefor duringoperation with it.

The magnet plate 54 is magnetized alternatively with north and southpoles on its down side and preferably comprises a large number of polepairs to provide a sufficiently high frequency for a frequency control,as is shown for example in FIG. 9 of the German OffenlegungsschriftDT-OS No. 2 533 187 or, respectively U.S. Pat. No. 4,174,484. FIG. 3 ina sectional view near 55 shows the kind of magnetization of the plate 54wherein as usual S means south pole and N means north pole.

Opposite to the magnetic plate 54 at the upper end of the bearing tube18 an isolating plate 57 is flanged in a recess 56. The isolating plateis provided on its upper side 58 with a meander winding 59 formed as aprinted circuit and the layout of which is indicated in FIG. 3. In FIG.3 for example the meanderwinding 59 has a total of 96 d radially runningsections 60, and the magnet plate 54 has 48 north poles 63 and 48 southpoles 64, that is 48 pole pairs. The meander winding 58 has a firstconnection 65 leading directly to a radially running section 60'. Thesection 60" neighboring section 60' runs to a compensation winding 66,which is led back on the inner side of the meander winding by about 320°to a second connection 67, which is located close by the connection 65.The wires 39 are soldered to the connections 65 and 67.

In case stray fluxes for example those from the stator winding 13 to 16permeate the meander winding 59 then they induce in the same aninterfering voltage u₁. At the same time they induce in the compensationwinding 66 an interfering voltage u₂ which about equals -u₁. Byconnecting the two voltages in series, which of course can be alsoperformed in another way, it is achieved that (u₁ +u₂) is about equalzero, that is at the output connectors the interfering voltage issubstantially suppressed and only the desired useful voltage isobtained, which is induced in the tachowinding 59 by the magnet plate54.

In the selection of the pole pair number p of the magnetic disc 54, andcorrespondingly also the number of the radially running sections 60 ofthe meander winding 59, one proceeds according to FIG. 4 withdetermining the output voltage u for various number of pairs ofmagneticpoles p with constant remaining air gap 68 width g and withconstant remaining frequency of rotation n. The curve 70 resulting has amaximum at p=p_(o) and for frequency control the area above the maximump_(o) is selected, since it is substantially required that the productof frequency and voltage has to be maximized, since with higherfrequency a smaller flicker effect results and thereby the control--andalso the ganging flutter--improve. This region is indicated in FIG. 4 asrange P_(opt) starting at po and extending to an upper limit number ofpoles.

In mounting, the rotor 44 is placed simply with its shaft 19 into thebearing tube 18. Then with the bolt 23 the air gap 68 of thetachogenerator 40 is positioned to for example 0.8 mm and then the motoris allowed to rest for a day. Thereby the trace spherical cap 20 pressesto some extent into the plastic disc 19' resulting in an increase in theoutput voltage of the tachogenerator 40 and the ganging flutter improveswhen the same is determined on the next day. According to experience thetrace spherical cap 20 sinks in successively a little more, that is theganging flutter improves with increasing age of the motor which ofcourse is very desirable.

A hollow cylindrical part 70 from plastic is disposed around the rotor44 such that it surrounds the rotor at a very small distance and therebyprevents the entering of foreign bodies into the motor 10. The part 70is provided with an extended foot 71, which is attached to the stator inthe manner shown.

For attachment to a chassis indicated at 72 serve side flanges 73 of thebasis plate 12. Mounting holes 74 are provided in these flanges. Thebasis plate 12 serves at the same time as a magnetic short circuit andas a mounting plate.

The molded plastic piece 24 forms in its middle a kind of protrusion 75for receiving the set screw 23 for the axial bearing 17. The protrusion76 forms a ring shoulder. The metallic set screw 23 is placed in theinjection mold before the injection molding of the molded plastic piece24 so that axially below its winding cannot be any plastic. Then byinjection molding at the same time in a simple fashion a thread isproduced for it. After the injection molding the set screw can onlyaxially be moved downward. The bearing tube 18 could carry at its lowerend a thread into which the set screw 23 is turned for support beforeinjection molding.

The present invention is particularly advantageous in connection with amotor according to German Offenlegungsschrift DT-OS No. 27 30 142 orrespectively, U.S. Pat. No. 4,211,963, since a so called meandertacho infact provides a small output signal, but a very good/ large ratio ofuseful to interfering voltage. In a construction according to the stempatent application the stator winding unavoidably generates a strongstray field, compare for example FIG. 2 of the present application,wherein alternatively the two coils 13 and 15 or the two coils 14 and 16are turned on simultaneously and wherein in each case the coils arepolarized equally so that for example upon switching on of the two coils13 and 15 both have on their upper side a north pole. (For betterscreening against this and other stray fields below the isolating plate57 an iron plate 80 can be placed as is indicated by dash-dotted linesin FIG. 1.)

Of course the invention can also be employed with motors having lesssizable stray fields, for example to motors according to the GermanOffenlegungsschrift DT-OS No. 2,533,187 or, respectively, U.S. Pat. No.4,174,484 and there especially in the stray field poor coil constructionaccording to the FIGS. 7 and 8, but also to the other coil arrangements,for example according to FIG. 2 of this patent application.

A relatively large distance is provided intentionally for the motor airgap between the permanent magnet ring 52 and the magnetic short circuitpart 12 (basis plate) in order to avoid that the axially directedmagnetic force becomes to great. It would otherwise press hard onto theaxial point bearing and effect there an increased shrinking and therebypossibly an undesirably large axial shifting of position of the rotor,and would also increase friction.

The distance has been selected such that the bearing load of the tracespherical cap 20 becomes not to large and on the other hand the motorstill brings the performance required. For this purpose the radius ofcurvature of the rounding of the motor shaft 9 is made as large aspossible.

This distance on the other hand allows a relatively large distancebetween the coil and the iron magnetic short circuit so that axiallybetween these, for example localized in a recess of the molded plasticpart 24, magnetic field sensors (for example one or two Hall-generators)can be disposed advantageously.

When the contact area for the trace spherical cap 20 of the shaft 9 ismade from a relatively inexpensive plastic for the larger injectionmolded part of the housing, for example the molded part 24, then theproblem is more severe. Therefor a polyurethane disc is provided asopposing part to the trace spherical cap, that is as a contact slidingelement. Such a high value plastic disc, which for example further maycomprise a molybdenum sulfide addition, is placed opposite to the tracespherical cap.

The large air gap allows a generous handling of the axial tolerances inmass production such that also a large air gap can be provided betweenthe rotor magnet and the disc coils of the stator, which air gappractically does not have to be considered regarding its manufacturingtolerance. Thus it is possible on the other hand to adjust the rotor forexample by the set screw 23 relatively accurate and to go exclusively bythe advantageously very narrow air gap of the tachogenerator requiredfor the tachogenerator. This results in a signal gain for thetachogenerator. The motor air gap is about twice as wide as the axialthickness of the stator coils. Initially the air gap of thetachogenerator is set to 0.8 mm, wherein by the shrinking during thelife of the motor by at most 0.3 mm an effective air gap of 0.5 mm isformed. Practically this is achieved by providing a so called contactbearing wherein the rounded end of the rotor shaft 9, which is at thesame time the rotor shaft of the tachogenerator, rotationally slidingruns against an axially adjustable central element, for example aplastic disc, with the rotor weight and the axial magnetical pullexisting between the magnetic short circuit elements 12 and 50. By thepressing according to Hertz at the contact area the axial rotor positionchanges long term and asymptotically against an end value, however intotal this is minimal.

What is claimed is:
 1. A tachogenerator for speed control of electricalmotors having an axial bearing for compensating the axial forces actingon the rotor comprisinga flat multipole permanent magnet solidlyconnected to the rotor of the motor and having a number p of pairs ofmagnetic poles on its surface uniformly arranged on a circle of diameterd for the pole centers, a circular wave winding solidly connected to thestator and separated from the permanent magnet by a flat air gap of awidth g and having a number p of angular substantially repeat units ofthe wave winding wherein p is equal to from about 0.5 to 1.5 times theratio of the diameter d divided by the width g, with d and g beingmeasured in the same units; and a corresponding return windingcorresponding to the wave winding and disposed running in oppositeangular direction to the wave winding for compensating the effect of thewave winding on a magnetic field component in axial direction runningthrough the circle of the wave winding.
 2. The tachogenerator accordingto claim 1 wherein p is equal to from about 0.8 to 1 times the ratio ofthe diameter d divided by the width g, with d and g being measured inthe same units.
 3. The tachogenerator according to claim 1 wherein thediameter d is at least about 10 mm.
 4. The tachogenerator according toclaim 1 wherein the diameter d is at most about 100 mm.
 5. Thetachogenerator according to claim 1 wherein the diameter d is from about30 mm to 50 mm.
 6. The tachogenerator according to claim 1 wherein thewidth g of the air gap is at least about 0.2 mm.
 7. The tachogeneratoraccording to claim 1 wherein the width g of the air gap is at most about2 mm.
 8. The tachogenerator according to claim 1 wherein the width ofthe air gap is from about 0.5 to 1 mm.
 9. The tachogenerator accordingto claim 1 wherein the number p of pole pairs is equal to from about 1to 2 times the number of pole pairs corresponding to the maximum inducedvoltage at the frequency desired.
 10. The tachogenerator according toclaim 1 wherein the wave winding is a meander winding.
 11. Thetachogenerator according to claim 2 wherein the meander winding isprovided by a conductor disposed on a plastic disc by photolithographictechniques.
 12. The tachogenerator according to claim 1 furthercomprising a soft magnetic material disposed on the side of the wavewinding opposite to the side facting the multipole permanent magnet. 13.The tachogenerator according to claim 12 wherein the soft magneticmaterial provides a magnetic shield.
 14. The tachogenerator according toclaim 1 wherein the wave winding is attached to a support tube of therotor.
 15. The tachogenerator according to claim 1 wherein the multipolepermanent magnet is attached to a soft magnetic backing solidlyconnected to the rotor.
 16. The tachogenerator according to claim 1wherein the multipole permanent magnet is a so-called rubber magnet. 17.The tachogenerator according to claim 16 wherein the so called rubbermagnet is adhesively attached to a soft magnetic material.
 18. Thetachogenerator according to claim 1 wherein the electric motor is a twostrand brushless d.c. motor.
 19. The tachogenerator according to claim18 wherein the rotor comprises successively a north pole magnet zone of120° electrical, a south pole magnet zone of 120° electrical and aremaining zone of 120° electrical having a magnetization generating in amagnetically active section of the stator winding at a relative motionsubstantially a zero voltage.
 20. The tachogenerator according to claim1 further comprisinga motor magnetic ring of the rotor forming in itsinterior a hollow space for disposing the tachogenerator.
 21. Thetachogenerator according to claim 1 wherein the flat multipole permanentmagnet comprises a magnetic plate formed from a rubber magnet.
 22. Thetachogenerator according to claim 21 further comprisinga magnetic shortcircuit plate attached to the magnetic ring of the rotor and wherein themagnet plate is adhesively attached to the magnetic short circuit plate.